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Re: Collections Protocol Review

From: Geoffrey M. Clemm <gclemm@tantalum.atria.com>
Date: Thu, 25 Feb 1999 15:41:07 -0500
Message-Id: <9902252041.AA06625@tantalum>
To: yarong@microsoft.com
Cc: w3c-dist-auth@w3.org

Judy: I think Yaron raised all of the issues that I had promised
you some backup material, so I'll use his message as a motivator for
finally getting it to you (:-).

   From: Yaron Goland <yarong@microsoft.com>

   The purpose of a spec is to help implementers and we should choose
   definitions that aid in that cause. Redirect references are based on 3xx and
   should be defined as such. This makes the situation extremely clear to
   anyone writing code.

I believe redirect references are *not* intended to be an abstraction that
means "things that return 3xx".  They *are* intended to be an abstraction
for "references that are visible to downlevel clients", and 3xx was a
convenient mechanism to do that.  So their semantics is determined by
"what is a good way to expose references to downlevel clients", and not
"always return 3xx".  Although the later semantics is simpler to implement,
it is a much less valuable abstraction to introduce (in my opinion).

   .... Continue to use Location and define an addition header
   LocationEx (a nod to my MS roots =). The definition is "First use Location
   and if that doesn't work then try the list of URIs in LocationEx." 100%
   compatible with existing systems and better functionality for advanced
   collection enhanced clients.

One of the advantages of a redirect reference is that it does not require
the server to hop off to other servers to see what's there (and gives the
downlevel client more control of where it wants the server to go hopping).
The LocationEx header loses these advantages.


   > > (Issue #5) Section 4.3.1 - Why DAV:reftarget, reftype and 
   > > refintegrity are
   > > stand alone properties - I do not understand why 
   > > DAV:reftarget, DAV:reftype
   > > and DAV:refintegrity are all defined as properties. As these 
   > > values help to
   > > define the nature of the resource type should they not have 
   > > their values
   > > included inside the DAV:reference element in the 
   > > DAV:resourcetype property?

I disagree.  The DAV:reftarget is much more like the "value"
of the resource, than it is the "type" of the resource.  The DAV:refintegrity
property could be orthogonal to each of these, in that it is easy
to imagine a server where the integrity properties of a reference
can be changed, without changing either the reftarget or the
direct/redirect "type" of the reference.

   > > (Issue #6) Section 4.3.1 - MKREF and the use of bodies - Why 
   > > doesn't MKREF
   > > follow the same rules that MKCOL has regarding the possible 
   > > inclusion of a
   > > body?

I agree.  I personally feel that all MKxxx methods should allow an
XML body that initializes any and all of the contents/properties of
the resource being created.  Just like a PROPPATCH.

   > > (Issue #7) Section 4.4.1 - Do references to collections have 
   > > to provide
   > > references for all the members of the collection?

No.  A reference is a member of a collection.
What collection would these "references to members" be part of?
The reference to the collection is not itself a collection, it is
a reference, so it cannot contain these "references to members",
and there doesn't seem to be any other candidate to do so either.

Or we could just look at the kinds of things references are there to model:
things like symbolic links in Unix file systems, and shortcuts in NT
(or symbolic links in NT, once they finally appear :-).  When you
create a symbolic link to a directory or shortcut to a folder, you
don't create symbolic links or shortcuts to all the members of that
directory or folder.

   > > - If one creates a
   > > reference to a collection is one also required to create 
   > > references to all
   > > the members of that collection? I suspect the answer is yes 
   > > but this is not
   > > clear from the specification.

The answer is no.

Judy: let's add words to the effect that "References to collections
allow clients to introduce multiple URL's for the same resource."  to
avoid this potential source of confusion.

   > > For example, the referential 
   > > resource in the
   > > example is http://www.svr.com/MyCollection/tuva/ and it points to a
   > > collection which has a member called history.html. Does this 
   > > mean that a GET
   > > on http://www.svr.com/MyCollection/tuva/history.html MUST 
   > > succeed?

If  http://www.svr.com/MyCollection/tuva/ is a reference to
http://www.other.com/YourCollection/, then
http://www.svr.com/MyCollection/tuva/history.html and
http://www.other.com/YourCollection/history.html are two different
URL's for the *same* resource.

So a GET on one will succeed if a GET on the other will succeed.
The server does the dereferencing for direct references.  The client
does the dereferencing for redirect references.

   > > In other
   > > words, that by creating a reference to a collection one is 
   > required to
   > > create references to its entire tree? Again, I suspect the 
   > > answer is yes
   > > (this would explain the PROPFIND response format for direct 
   > > references as
   > > well as the COPY behavior for direct references) but this 
   > > requirement is
   > > never explicitly stated in the spec.

You are assuming that every distinct URL selects a distinct resource.
This is not the case, and references to collections allow clients
to explicitly make this not be the case.

   HTTP defines that there are HTTP URLs and that HTTP URLs point to resource.

Yes.

   If creating a reference called http://foo/bar that points to a collection
   with a member baz and thus I can perform a GET on http://foo/bar/baz, even
   though the MKREF only created http://foo/bar, then there exists a resource
   called http://foo/bar/baz

Yes.

   and it is a reference.

No.  References are not in HTTP-1.1, so there's no way it could require that
this resource be a reference.

   Thus the only possible
   conclusion is that creating a reference to a collection causes references to
   be created to all the members of that reference's entire tree. Otherwise
   http://foo/bar/baz would fail.

No, http://foo/bar/baz is a resource that just happens to have another URL
that can be used to refer to it.

   > > (Issue #9) Section 4.6 - Banning passthrough behavior on 
   > > DELETE and MOVE - I
   > > have a direct reference http://foo/bar to http://bar/blaz. I 
   > > can define a
   > > MOVE on http://foo/bar to http://icky/bik as meaning that 
   > > http://bar/blaz is
   > > to be moved to http://icky/bik.

True.

In the presence of references, there are always two possible
interpretations of every request, namely, apply it to the reference
itself, or apply it to the target.  For methods where both of
these interpretations are useful, the question is not which to
support (we will support both), but rather, which interpretation
is the default and which interpretation  requires Adv.Col. methods
or properties to achieve.

The desired behavior for downlevel clients will significantly affect
the choice of a default.  Note that it will *affect* the choice of a
default, but will not determine it, since there could be uplevel
considerations that override the downlevel ones.

For most methods (e.g. GET, PUT, LOCK), the only way to make the results
of the method visible to a downlevel client is to have it affect
the target.  This is actually unfortunate for "write" operations, because
the downlevel client can unknowingly affect what is seen at a
whole bunch of other URL's.  (Yaron mentions this below).

This would be *especially* unfortunate for heavyweight operations like
DELETE and MOVE, which can result in all references to those other URL's
to dangle, the bane of Web management.

Happily, we can fulfill the semantics of DELETE and MOVE for downlevel
clients *without* the damaging effect on these other URL's, by having
the default behavior of the method be "no-passthrough".  A client using
the URL(s) explicitly named in the DELETE and MOVE will see the desired
effect, without trashing all the other references to the target of
that reference.

Now an uplevel client can find out the URL of the target, and do the
DELETE/MOVE on it (knowingly trashing all references to that target).
It requires them to do an explicit PROPFIND, and is not something
downlevel (or uplevel!) clients end up doing by mistake.

   > > Following the logic discussed 
   > > in Issue #7 I
   > > can even properly define how to move any children that 
   > > http://bar/blaz might
   > > have. With DELETE I can define a DELETE of http://foo/bar as 
   > > meaning that
   > > http://bar/blaz should be deleted. So there doesn't seem to 
   > > be any technical
   > > reason to ban passthrough behavior on MOVE and DELETE 
   > involving direct
   > > references.

Agreed.  It is a downlevel usability argument that bans it.

   > > 	I suspect the real reason that passthrough behavior is 
   > > disallowed on
   > > direct references is because of concerns regarding interactions with
   > > existing WebDAV clients. In most file systems that support 
   > > links if one
   > > deletes or moves a link, only the link is deleted or moved. I 
   > > suspect the
   > > authors are concerned that allowing passthrough on DELETE and 
   > > MOVE would
   > > mean that if an existing WebDAV client asks for a DELETE or 
   > > MOVE then the
   > > resource being pointed at would be deleted/moved as well, 
   > which wasn't
   > > something the non-reference enabled client would have 
   > > intended.

Exactly.

   > >  As such I
   > > think it is completely reasonable to design the protocol such 
   > > that if a RFC
   > > 2518 client issues a DELETE or MOVE then only the direct 
   > > reference and not
   > > the target is affected.

Having it do one thing for 2518 clients, and another thing for other
clients would be very confusing, and cause all sorts of mistakes.
(Or did you not intend to imply that we would do something different
for non-2518 clients?)

   > > In fact I would propose the design 
   > > rule that when
   > > moving/deleting a resource the resource should retain its 
   > type unless
   > > explicit instructions to the contrary are given.

Agreed.

   > > That is, 
   > if you are a
   > > direct reference at the source then you should be a direct 
   > > reference at the
   > > destination unless explicit instructions to the contrary 
   > are provided.

Agreed (for a MOVE operation).

   > > 	The way to easily implement this rule is to give 
   > > no-passthrough a
   > > value and define the default for that value on a method by 
   > > method basis.
   > > PROPFIND and COPY, for example, would default to no-passthrough: f.
   > > MOVE/DELETE would default to no-passthrough: t.
 
Sounds fine to me (as long as these rules are not HTTP-level dependent).

   > > 	The previous all applies to direct references. Redirect 
   > > references
   > > are obviously a different animal since one can only directly 
   > > manipulate the
   > > reference and never the target. Thus the only issue is - 
   > > should a 302 ever
   > > appear in a COPY/MOVE/DELETE response? Following the law of 
   > > minimum surprise
   > > an RFC 2518 client getting a 302 would be very surprised. 
   > > They ordered that
   > > all resources be copied, redirect references are resources, 
   > > therefore they
   > > should be copied. As such I propose that redirect references on
   > > COPY/MOVE/DELETE always behavior as if no-passthrough equals 
   > > t, regardless
   > > of its actual value.

For MOVE/DELETE, yes.  This is consistent with direct reference
functionality.  For COPY, I and Judy vote no.  The client needs to be
warned that there really will *not* be a copy made, and it will then
have sufficient information to make a real copy if it needs to.

   I still fail to see why you are banning perfectly reasonable functionality
   when there does not appear to be any technical or protocol related reason
   for doing so.

We are not banning functionality, we are just picking the other more
expected functionality as the default.  Remember, both interpretations
are valid and are supported -- we're just deciding what is the default.

   As my comments demonstrate it is completely possible to use
   DELETE and MOVE to effect the targets and to do so in a manner which causes
   the expected behavior with RFC 2518 clients.

Only if you have one set of clients (2518) clients see one kind of
behavior and other clients see a very different kind of behavior.
Since you can achieve the alternative DELETE/MOVE semantics, why would
you consciously introduce this kind of inconsistency?

   As such what possible reason
   can there be for banning this logical and well defined behavior? Just
   because it doesn't fit your conceptual model? This would argue for changing
   your model not banning this functionality. Again, I call upon the general
   HTTP rule "allow unless you have a damn good reason for not doing so."

We're not banning anything, we're just making sure the default
is consistent and corresponds to the semantics that the majority of
servers and clients support/expect.

   > > (Issue #10) Section 4.7.1 - Passthrough locks on direct 
   > > references - I'm
   > > thoroughly confused regarding why a passthrough lock doesn't 
   > > lock both the
   > > reference and the target.

This is the main point I owed Judy a writeup on.  There are a variety
of reasons why it should only lock the target.  Many of them have to
do with making the semantics of a LOCK work in the presence of versioning
(and versioned namespaces in particular).

But there are some simple examples that don't require understanding
the needs of versioning, so let's see if those are convincing first.

For example, if I take out an exclusive lock on /x/y/z.html, and
/x/ is a reference, then I would need to take out a lock on /x/
or else someone could just DELETE /x/, and put a new reference or
collection in its place, thereby causing a different resource to
be selected by /x/y/z.html.

But that means that any subsequent attempt to get an exclusive lock
an *any* resource with a URL beginning /x/ will fail, because of
the prior exclusive lock on /x/.

This kind of behavior is even more disastrous in the presence of
versioned URL namespaces, where the whole point is to allow
multiple people to use the same URL (combined with a Version-Name
or Workspace header) to access different resources.

The alternative, which says that a lock on /x/y/z.html locks the
*resource* that this URL refers to, but does not lock the binding
of that URL to that resource, avoids this problem, and furthermore,
is consistent with access control on file systems (those file systems
guys were actually pretty smart when they decided on these kinds of
issues).

   > > I read the spec a couple of times 
   > > and I still
   > > don't get it. For example, imagine an RFC 2518 client 
   > issued a lock on
   > > http://foo/bar which is a direct reference to
   > > http://bar/blaz. To the RFC 2518 client it appears that they have 
   > > locked http://foo/bar which will act exactly as 
   > http://bar/blaz. This
   > seems > the much more reasonable behavior.
   > > I'm sure there is a scenario here I'm missing but whatever 
   > it is, I wasn't
   > > able to discern it from the spec. I apologize if the 
   > scenario is staring
   > me
   > > in the face and I am just failing to see it.

No, the scenario (especially the versioned namespace scenario) is
not staring you in the face (unless you've ever tried to implement
a versioned namespace!).

   The current situation is, in my opinion, a clear violation of RFC 2518. RFC
   2518 is crystal clear that locking a resource means that the resource is
   locked.

Not at all a clear violation.  A URL is a handle on a resource, it is
not the resource.  This is made very clear by the fact that multiple
URL's can refer to the same resource.  So if 2518 said "you lock a
URL", then it would be crystal clear, and I would have objected
strenuously to the spec, because that definition of locking is
incompatible with advanced versioning, where users must have the
ability to use the same URL (plus a header) to refer to different
resources.

No, 2518 got it right.  What you are locking is a resource, not the
binding of a URL to a resource.

   Your current definition does not do this. In fact someone locking a
   direct reference, which actually locks the target, could end up with the
   reference changed underneath them, thus leading to the violation.

The alternative is worse (i.e. having a lock on one resource preventing
you from locking any other resource in that tree).  This is why a "lock"
on a file in a file system (e.g. making it read-only except for the owner),
does not "pin" that inode to that pathname.  Again, those file system
guys knew what they were doing.

   Whatever
   solution you choose the reference MUST be locked or this specification is in
   violation of RFC 2518 and must therefore choose a new method name rather
   than LOCK.

   Personally I think you should just accept that LOCKing references can be a
   bit wacky and will result in locking both the reference and the target.

We do accept that LOCK'ing references will be a bit wacky, and
they will lock only the target (:-).

   > Thanks again for (as always) prompt and insightful comments.

Me too!!

Cheers,
Geoff
Received on Thursday, 25 February 1999 15:41:20 GMT

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